| Nowadays, with the rapid development of urea production industry, considerable water is being used, at the same time, much urea wastewater discharged from its producing processes are making the environment pollution become an outstanding problem. In order to enhance the utilized efficiency of water resource and realize the sustainable development, urea wastewater treatment and reuse are paid more attention by urea production industry. However, the effluent of current wastewater treatment processes can't match the quality of water used in urea production because of the characteristics of urea wastewater, such as containing much pollutants, low organic materials, high urea concentration and fluctuant quality and quantity of wastewater etc.According to the characteristics of high urea wastewater and the present status of urea wastewater treatment technologies, the primary experiments of urea removal with membrane separation, Fenton oxidation and superior bacteria oxidation were conducted, respectively, in order to find an effective and convenient treatment technology of high urea concentration wastewater. The results of experiments showed that there were more advantages with biological treatment than that of others. Thus, the combined process centered on Biological Aerated Filter (i.e. BAF) was proposed to treat high urea concentration wastewater, and its characteristic was studied at pilot-scale in Taxinan Petrochemical Plant.The start-up of BAF was used artificial cultivated high activated sludge and the biofilm in BAF was cultivated by the method of circular cultivation. The results of experiments showed that startup process of BAF was concerned with the aeration type, media type and operation mode. Pre-aeration (3~4 d) and higher filtration rate could short startup period of BAF; activated carbon media could accelerate the startup compared to ceramic media.The major influence factors of BAF in treating high urea concentration wastewater were substrate load, pH value and hydraulic load etc. The results showed that when the urea load, ammonium load and pH value were at 0~23 kg/(m~3·d), 0~3 kg/(m~3·d) and 7.0~11.0, respectively, all of them could not affect the urea removal efficiency markedly, and its mean urea removal efficiencies were all above 90%. While the highest removal efficiency was obtained when the COD load and hydraulic load were 0.87 kg/(m~3·d) and 2.1 m~3/(m~2·h), respectively. In addition, when the max free ammonia concentration was 64.48 mg/L, it could not inhibit the hydrolysis reaction of urea obviously.The urea was removed mainly by urease produced by microorganism in BAF, the superior bacteria of urea hydrolysis were heterogeneous, while some sorts of nitrifying bacteria also had a certain ability to hydrolyze urea. By analyzing the variations of total biomass and their activities in BAF, it showed both of them decreased along with the filtration bed height in BAF, and the biomass and total activity in activated carbon media BAF were three times of the ceramic media BAF.Kinetic equation of urea removal was concluded based on the structure of BAF and M-M equation, and the kinetic rule of urea removal in BAF was analyzed too. The experiment results showed that plug flow structure of BAF leaded to the different layers of biomass, activity, and substrate. Neither zero order kinetics nor first order kinetics model could be used to express the kinetic process of urea removal, but the combination of both. It was preferred to fit urea removal process in BAF by gradual-subsection.Backwashing process is the key step to ensure high treatment efficiency of BAF, concept of optimal backwash cycle was put forward and formulized from the view of enhancement of BAF purification effect. In addition, based on the equation and considering the head loss, appropriate backwash parameters were determined. Under the appropriate backwash condition, the removal of urea and ammonium nitrogen in BAF could achieve their original level before backwash after operating 1 hour and 2 hours, respectively.Finally, based on the characteristics of mixed wastewater discharged from Taxinan Petrochemical Plant and the aim of wastewater reuse, the combined process of BAF-Sand filtration-Ion exchange was proposed and an initial economic analysis was carried on.
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